1. Field of the Invention
This invention relates to a conductor covered with a clear flame retardant composition, and, more particularly, to an extrudable clear flame retardant polyvinyl chloride composition having flexible properties to facilitate the use thereof to jacket telephone cordage and having protection against degradation by ultraviolet or diffused light.
2. Description of the Prior Art
Most telephone users are familiar with what is referred to in the art as the line or mounting cord which extends the telephone circuit from a connecting block, either floor or wall mounted, to a telephone set. The telephone set consists of the housing and the handset which is connected to the housing by a retractile cord. It has been the custom to color match all of the aforementioned telephone components, that is; the line cord, the housing and handset and the retractile cord. Due to the large number of colors and the several different lengths of cords that are available, installers must maintain an uneconomically large inventory of line and spring cords on service vehicles in order to provide the many combinations of length and color. This large mobile inventory results in excessive inventory holding costs.
To reduce the excessive inventory holding costs, studies were made to ascertain if a single color line cord, which would significantly reduce the number of combinations, could be used. It was decided that this universal color must be neutral in any environment, compliment the telephone set, be aesthetically appealing and be widely accepted by telephone subscribers. At first, a satin silver colored polyvinyl chloride (PVC) jacket composition was selected as the universal color for line cords. Sample cords were manufactured and installed in a product test area, but the acceptability was less than that which had been hoped for.
Subsequently, it was decided to field test a line cord having tinned tinsel conductors individually insulated with a clear nylon material and jacketed with a clear polyvinyl chloride jacket. The tests showed that a cord of this construction and color had aesthetic appeal as indicated by a high acceptance rating in the product test area.
However, although the use of a non-flame retardant clear polyvinyl chloride composition solved the question of what universal color the consumer would accept, other problems arose due to the shortcomings of the clear PVC composition that had been employed.
Clear flame retardant polyvinyl chloride compositions possessing characteristics required for telephone line cord jackets, are not commercially available. To obtain flame retardant characteristics, the prior art approach has been to employ additives such as antimony trioxide. This is sufficient if one is working with opaque formulations but the addition of additives such as antimony trioxide to a clear polyvinyl chloride composition is destructive of clarity. In the case of antimony trioxide, the material becomes milky white in color.
Of course, the composition which is sought must be such that after a clear line cord having acceptable flame retardant properties has been successfully manufactured, the composition remains clear. In this regard, the exposure of the line cord to ultraviolet radiation in sunlit rooms tends to degrade the color of plastic covered cords.
Therefore, it is an object of this invention to provide a conductor covered with a polyvinyl chloride composition which is flame retardant, which is of a clear color, and which is stabilized against degradation by ultraviolet or diffused light. These characteristics are essential to prevent discoloration of the PVC composition in the countless environments in which telephone sets are used.
Yet another object of the proposed invention is to provide a conductor covered with a clear flame retardant PVC composition that possesses self-extinguishing characteristics.
The desire to render inside wiring flame retardant is not without precedent. For example, in the production of textile-served wire which is particularly adapted for use in distributing frames of telephone exchanges, a polyvinyl chloride insulating layer, which includes a suitable flame retardant plasticizer, is extruded over a conductor wire. A textile serving is wrapped over the insulation whereupon the served wire is passed through an impregnation chamber wherein a substantially transparent lacquer coating is applied under pressure to protect the serving.
In an application filed on July 25, 1968, Ser. No. 747,757, now U.S. Pat. No. 3,668,302, there is disclosed a lacquer composition for impregnation of the cotton and which possesses superior flame retardant characteristics. The composition includes a cellulose acetate base, a derivative of cresylic acid and phosphorous oxychloride, as the plasticizer, a brominated monomeric triallyl phosphate as the flame retardant and a solvent.
Of course, while the aforementioned composition is useful in the environment disclosed with the accompanying manufacturing problems associated therewith, the search for a clear flame retardant polyvinyl chloride composition for jacketing line cords demands a composition ideally suited to the specific needs of line cords per se.
To be rated self-extinguishing, the composition must not support combustion under normal atmospheric conditions. In terms of A.S.T.M. under designation D2863, the subject composition should meet a minimum limiting oxygen index (L.O.I.) of 26. The oxygen index is defined as the volume percent of oxygen required to support combustion. The greater the limiting oxygen index, the better are the flame retardant properties of the composition.
Of course, as in the planning of every product, it is necessary to weigh the costs involved. Phosphate materials, which are traditionally of a flame retardant nature, are expensive. Care must be taken to optimize flame retardancy, but at the same time consideration must be given the costs so as not to unduly burden the customer.
The development of a composition is not uncomplicated further by additional demands because of the innumerable environments in which telephones are installed. For example, the final jacketing composition must be flexible and have superior low temperature flexibility properties so that the jacket will not be embrittled in some of the expected environments of use, such as in office buildings prior to placing the heating system in service.
In addition, the clear flame retardant jacketing composition must also be of such a nature as to not mar or damage any lacquered surface such as a desk or table top. If the plastic jacket in contact with such a surface does not possess adequate mar resistance the jacket surface will be permanently damaged.
As can be surmised from the discussion henceforth, there is sought a unique combination of constituents that will provide a telephone cord material tailored to the needs of the customer while still within the bounds of manufacturing economies.